In the time of Theodosius, at the end of the 4th century, orthodox Christians throughout the Roman empire seem to have good reason for optimism. The very existence of the vast Roman empire is seen now as the ordained precondition for the triumph of Christianity. As a writer of the time puts it, 'the Roman peace has prepared the road for the coming of Christ'.

A century later the outlook is very different. The first shock has been the sack of Rome itself in 410 by a horde of barbarians who are also Christians. Admittedly they are heretics, for the Visigoths under Alaric subscribe to Arianism. But even so! The advent of Visigoths in Rome partly prompts St Augustine's most influential book, the City of God.

The City of God: AD 413-426

Augustine's great theological treatise takes as its starting point accusations levelled against Christians after the disaster of AD 410. The walls of Rome are breached in this year, for the first time in eight centuries, by barbarian hordes - the Visigoths of Alaric. How has this come to be?

Some are grumbling that it is because the old pagan gods of Rome have been set aside in favour of Christianity. Augustine's answer is unapologetic. The sack of Rome is unimportant, he argues, because it is only an earthly city. What matters is the city of God.

This profound distinction is one which he would certainly have made regardless of the events of 410, for the contrast is not a simple one between a real city on earth and an imagined city in heaven. Augustine is contrasting the combined body of true Christians, both alive and dead, on one side; and pagans and heretic Christians, alive and dead, on the other. The former, representing the city of God, are indestructible. They will live for ever with God. The rest, in the final destruction, will perish.

The two groups are implacable enemies - devils against angels up there, the evil against the good down here. The battle cannot be won in the short term. It must be accepted, in the certainty for God's people of ultimate victory.

At the heart of this theology there lurks a stark concept which Augustine does nothing to soften. God's people, the elect, are a clear group, eternally distinct from the damned. But when and how does one join the elect? God foresees all, so he must know even before the beginning of a human life whether that person is to be one of his. Does that mean there is nothing the individual can do to influence his fate?

It does indeed according to Augustine - though others, less rigorous, may argue that what God foresees could be the result of human choice. The harsh creed of predestination is bequeathed by Augustine to other like-minded Christian leaders, among them most notably Calvin.

Nestorius and Nestorians

The Christian church's decision on the Arian heresy, at Nicaea in 325, brings in its wake further problems of the same kind. If Jesus is 'of one substance with the Father', he is incontrovertibly God. It follows that the Virgin Mary, though herself entirely human, gives birth to God. The phrase theotokos ('bearer of God' in Greek) is soon widely used of her.

This becomes a political issue in the 5th century when Nestorius, the bishop of Constantinople, says that the word theotokos should not be used. An ecclesiastical rival, Cyril the patriarch of Alexandria, seizes the opportunity to harm Constantinople by declaring its bishop a heretic.

The controversy rapidly spreads, until a council is convened at Ephesus in 431 to consider the matter. At this council Nestorius (refusing to attend in person) is outmanoeuvred by the unscrupulous Cyril, who secures his rival's removal from his post at Constantinople. Exiled to a remote spot in southern Egypt, Nestorius dies in about 451 - the year in which another council, at Chalcedon, confirms the decisions of Ephesus and specifically declares that Mary is indeed theotokos, the Mother of God.

In certain regions, particularly Syria, there is widespread support for Nestorius. Christians of this persuasion, escaping from militant orthodoxy, move east into Persia.

The Persian empire, with Zoroastrianism as its religion, is tolerant of Christianity. An existing Christian community has evolved a tradition of doctrinal independence, being cut off politically from the Byzantine empire. The Persian Christians now accept the doctrine of the Nestorians arriving among them. In effect a separate Nestorian church develops.

After the arrival of the Muslims in Persia, in the 7th century, the Nestorians are even more cut off from the Christian world to the west. But they prosper and expand, along eastern trade routes, with the result that the first Christians to reach many regions of the east are technically heretics.

Nestorian Christianity spreads from Persia to India. It reaches China, moving east (like Buddhism before it) along the Silk Road. And when western travellers reach the Mongol city of Karakorum in the 13th century, they find Nestorian priests living there. Thereafter Nestorian influence declines, particularly when other Christian missionaries arrive in the east. But various small Nestorian communities survive in the 20th century, some of them now linked with the Roman Catholic Church. Few Nestorians (and not even Nestorius himself) have believed the extreme version of the heresy as defined by his enemies - that the incarnate Christ was a human being somehow made special by God.

The western empire: c.500

By the end of the 5th century only the eastern part of the empire, from Constantinople round the Mediterranean to Alexandria, is a stable area of Christian orthodoxy. The situation in the west is very different. The pope and his entourage in Rome survive as an orthodox enclave within an Italy dominated by the Arian Ostrogoths. Spain is in the hands of Arian Visigoths. North Africa is controlled by Arian Vandals (see the Spread of Arianism).

Christians in Gaul and Britain are in an even worse predicament. They struggle to keep the faith in Roman territories encroached upon by pagan Germanic tribes, untouched even by Arian Christianity - the Franks in Gaul, the Angles and Saxons in Britain.